Attenuators are used in applications that require signal level control. Level control can be accomplished by either reflecting a portion of the input signal back to its source or by absorbing some of the signal in the attenuator itself. The latter is often preferred because the mismatch which results from using a reflective attenuator can create problems for other devices in the system such as nonsymmetrical two-port amplifiers. It is for this reason that absorptive attenuators are more popular, particularly in microwave applications. The important parameters of an absorptive attenuator are: attenuation as a function of frequency; return loss; and stability over time and temperature.
It is known that variations in temperature can affect various component parts of a microwave system causing differences in signal strengths at different temperatures. In many cases, it is impossible or impractical to remove the temperature variations in some Radio Frequency (RF) components. For example, the gain of many RF amplifiers is temperature dependent. In order to build a system with constant gain, a temperature-dependent attenuator is placed in series with the amplifier. The attenuator is designed such that a temperature change that causes the gain of the amplifier to increase will simultaneously cause the attenuation of the attenuator to increase such that the overall gain of the amplifier-attenuator system remains relatively constant. However, prior art temperature-dependent attenuators do not offer the bandwidth needed for certain wideband applications.
Voltage controlled attenuators (VCAs) are a fairly common element of almost any RF or microwave circuit. Their function is to change the amplitude of a signal based on some external signal, usually a voltage or current. A common use is the leveling of a signal so that both strong and weak signals can be adjusted in amplitude to provide a constant level signal to the next stage of the circuit. Another use is the balancing of multiple signal paths so they all have the same gain. A third use would be to use a VCA to control the gain of an amplifier over temperature by varying the control voltage based on a measurement of the ambient temperature. This last use is to counter undesired changes to the gain of the amplifier when the ambient temperature changes.
The vast majority of presently available VCAs include either diodes, transistors, or FETs (field effect transistors). These active devices have non-linear transfer characteristics which result in distortion to RF and microwave input signals. This causes additional and unwanted signals to be generated which are not present in the original signal. These additional signals have the potential of causing interference to other services, like police or fire departments that use the same frequencies as the additional signals.
U.S. Pat. No. 5,332,981, issued to Joseph B. Mazzochette, et al., issued Jul. 26, 1994, entitled “Temperature Variable Attenuator,” which is incorporated herein by reference, describes an attenuator that includes temperature variable resistors (thermistors) in the attenuating path. As shown in FIGS. 1A and 1B which are reproduced from FIGS. 1 and 3 of the '981 patent. conventional attenuators include a Tee attenuator 10 comprising a pair of identical series resistors R1 and a shunt resistor R2 and a Pi attenuator 12 comprising a series resistor R2 and two shunt resistors R1 and R3. FIG. 1C is a plot reproduced from FIG. 2 of the '981 patent, showing a family of constant attenuation curves from 1 to 10 dB and a constant 50 ohm impedance curve descending from the upper left of the plot to the lower right. The vertical axis on this plot represents the value of shunt resistor R2 in the T attenuator 10 and the horizontal axis represents the values of series resistors R1. The point of intersection between the 50 ohm impedance curve and an attenuation curve gives the value of R1 and R2 that produce the attenuation represented by the attenuation curve and a 50 ohm impedance match.
In the temperature variable attenuator of the '981 patent, the temperature coefficient of resistance (TCR) of at least one resistor is different such that the attenuation of the attenuator changes at a controlled rate with changes in temperature while the impedance of the attenuator remains within acceptable levels.
While such prior art temperature-dependent attenuators have enjoyed considerable success in many applications, they do not offer the bandwidth needed for certain wideband applications.